1. Field of the Invention
This invention generally relates to imaging technology, and, in particular, to transfer-type electrophtographic copying machines having a phantom image suppression function. More specifically, the present invention is concerned with electrophotographic imaging machines including a blanket exposure device for carrying out a blanket exposure prior to the transfer of a toner image formed on an imaging surface to a recording medium and a discharging device for discharging the remaining charge on the imaging surface after the image transfer.
2. Description of the Prior Art
Electrophotographic technology is well known in the art and a typical electrophotographic copying machine of the image transfer type includes a photosensitive drum comprised of a photosensitive member attached to the peripheral surface of a rotatably supported drum with the outer peripheral surface of the photosensitive member defined as an imaging surface, a charger for uniformly charging the imaging surface, an image exposure device for exposing the thus uniformly charged imaging surface to an original image thereby having the charge on the imaging surface selectively dissipated to form an electrostatic latent image, a developing device for developing the latent image thereby converting the latent image into a visible toner image, a blanket exposure device for applying uniform light to the toner image on the imaging surface, an image transfer device for transferring the toner image to a transfer medium from the imaging surface, and a discharging device for removing residual charge on the imaging surface after the image transfer by the application of uniform light to the imaging surface.
As described above, the blanket exposure device is provided so as to make the difference in potential between the imaging surface and the toner image to be as small as possible thereby allowing to enhance the transfer characteristic of the toner image to the transfer medium from the imaging surface and the separation characteristic of the separation of the transfer medium from the imaging surface. On the other hand, the discharging device, normally comprised of a lamp, for removing residual charge from the imaging surface is provided to prepare the imaging surface ready for the following sequence of copying cycle. Particularly, when A.C. discharging is employed in the separating device for separating the transfer medium from the imaging surface, the blanket exposure device is provided so as to attain desired characteristics for the image transfer and the separation of transfer paper.
This blanket exposure prior to image transfer has its object to have the charge in the imaging surface at the position below the toner image removed, so that the amount of light required for this blanket exposure tends to be larger necessarily. For example, if desired to maintain only the characteristics of image transfer and separation of transfer medium, the amount of light of blanket exposure prior to image transfer approximately ranges between two to seven times of that required in image exposure. However, the fatigue characteristic of photosensitive member comes to be important when such a large amount of light is used. That is, a phantom image may be produced due to differences in fatigue levels between the image and non-image portions on the imaging surface. On the other hand, since the discharging lamp is to simply remove residual charge from the imaging surface after image transfer, the required amount of light is not so large and it may be less than the amount of light required in image exposure.
Customarily, the light amounts for the pre-transfer and post-transfer blanket exposures have been determined separately and thus there has been a chance of producing a phantom image due to differences in light amount between these two blanket exposures. Depending upon which of the light amounts is exceedingly larger than the other, a negative or positive phantom image may be produced.